Speed control of induction motors



Dec. 11, 1928.

H. K. SCHRAGE SPEED CONTROL OF INDUCTION uo'rons 2 Sheets-Shem Inventor Hidde KSchr'age, I y

Original Filed April 8, 1.925

Hi5 Attorn eg.

Dec. 11. 1928.

H. K. SCHRAGE SPEED CONTROL OF INDUCTION MOTORS 2 Sheets-Sheet Original Filed April 8, 1925 Fig.5.

Inventor": H idde K. Schr'age, by

H is Attorney.

Patented Dec. 11, 1928.

UNITED STATES PATENT OFFICE.

HIDDE K. SCHRAGE, OF PARIS, FRANCE, ASSIGNOR T0 GENERAL ELECTRIC COMPANY,

A CORPORATION OF NEW YORK.

SPEED CONTROL OF INDUCTION MOTORS.

Application filed April 8, 1925, Serial No. 21,662, and in France June 20, 1924. Renewed August 14, 1928.

My invention relates to systems wherein the speed of an induction motor is controlled by variation in the magnitude of a regulating voltage applied to its secondary circuit either through a regulating winding forming a part of the motor, or through a regulating machine interconnected with the motor, and has for its object the provision of an improved arrangement for. controlling the commutation of the regulating machine or winding in a manner to render possible the utilization of regulating voltages higher than those heretofore used for this purpose.

It is well known that the speed of an induction motor can be regulated by means of a variable voltage applied to its secondary circuit, and that the limit to which the regulating voltage can be increased is dependent on the commutation characteristics of the regulating winding or machine through which this voltage is applied to the secondary circuit of the motor. If the motor be supplied with current at a frequency of 60 cycles, for example, difficulty is encountered, when the Voltage at the commutator of the regulating winding or machine exceeds 45 or 1 50 volts, due to the large commutator and great number of commutator brushes required to transmit the large regulating currents required at these voltages. Inaccordance with my invention, provision is made for suppressing the field flux in the commutation zone of the regulating winding or ma chine.

My invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring to the drawings, Figs. 1 and 5 show speed control arrangements in which my invention may be embodied; Figs. 2 and 3 show various details of a regulating machine constructed in accordance with myinvention; and Fig. 4 shows an arrangement wherein the regulating element and the mo tor are embodied in a single structure.

Fig. 1 shows a speed control aggregate or system wherein a regulating machine 1 is provided with a commutator, the brushes 8 of which are interconnected with the secondary circuit of an induction motor 2 and the slip rings 7 of which are connected through a transformer 3 to a polyphase line 4 from which current is supplied to the primary circuit of the motor 2.

Assuming the motor 2 and regulating machine 1 to'have the same number of poles and to be operated at the same speed, the transformer 3 may be adjusted in a well known manner to vary the voltage applied to the secondary circuit of the motor by the frequency changer or regulating machine 1. As indicated above, the upper limit of the regulating voltage is rather low unless special means are provided for improving the commutation of the regulating winding or machine. Thus, assuming the rotor winding of the regulating machine to be a multiple winding with one turn per commutator bar, the voltage per bar hereinafter designated as e, is equal to where f=the frequency of the polyphase line 4, f =the speed of the motor, f the slip frequency of the motor, and ==the flux embraced by a turn of the regulating machine rotor winding. It will be readily understood that this voltage is dependent both on the speed at which the winding or machine is rotated and on the rotational speed of the field flux. The voltage per commutator bar may therefore be divided into a component 61 7T'\/ f110 representing the part of the total voltage 6 produced by rotation ofthe regulating winding or machine and a component due to rotation of the field flux.

If no means are provided for limiting the field flux in the commutating zone of the regulating machine or Winding, the total voltage 6, which is equal to ed-e is necessarily limited to a value at which good commutation is realized. It is possible, however, to suppress the field in the commutating zone by slotting the stator field structure at points adjacent the commutated conductors of the regulating winding or machine. In this manner, the component voltage 6 in the commutating zone may be suppressed and both the flux and the voltage 6 may be increased in proportion to the quotient obtained by dividing f into 7. Thus assuming a speed regulation of plus and minus 30% and current to be supplied to the motor at 50 cycles, it is possible to raise the voltage at the commutator of the regulating machine from $5 volt-s to X1 5) +30=150 volts and to reduce the size of the commutator and the number of commutator brushes accordingly.

Fig. 2 shows certain details of a frequency changer or regulating machine wherein a stator member 5 slotted at points adiaccnt the commutation; zones of a rotor winding 6 connected to slip rir 7 and to commutator brushes As incicated by Fig. l, the brushes 8 may be connected to the secondary circuit of the motor 1 and the slip rings 7 may le connected to the line 4 through the transformer Damper windings may be placed in the slots of stator member 5 in order more effectively to suppress the flux in the zone of commutation. ll hile the winding 6 is diagrammatically shown in Fig. 2 as a ring winding, it will be evident that this winding usually will he of the drum type and that. in the case of three phase machines. it may be wound with a pitch of 120 electrical degrees. In order to minimize deformation of the regulating mach ne iield by the slots in the rotor structure. it desirable that these slots be made as small as is consistent with good commutation. Inductance coils 11 may be connected between the slip rings T and the transformer 3 to minimize the effect of harmonics resulting from deformation of the field.

Fig. 3 shows a regulating element comprising two frequency changers coupled to the same shaft and connected to have current transmitted between tl eir rotor windings (i and 6' and the line -l through a single set of slip rings 7. In utilizing the regulating element of Fig. to control the secondary voltage of the motor 2, i is necessary that the secondary windings be independent of one another or, in other words, to provide the motor with six slip rings. as shown for exa .iple in Fig.2. 1 of British Patent 16 ;359 of 1909, each connected to a diti'erent terminal of the three phase secondary WllKllDQj.

Assuming the brushes 8 and 8 to be in correspondin positions on the windings 6 and 6 respectively. and we irrcsponding brushes of the two machines to be connected to oppo site ends of the same secondary windings, the regulating voltage may be varied by shift the stator 5 and brushes 8 in one direction while shifting the stator 5 and brushes 8' in the opposite direction and through an angle equal to that through v rich the brushes 8 and the stator are shizted. The power factor of the motor 2 may be controlled by shifting the stators and brushes of the two machines in opposite directions through unequal angles.

Fig. 1- shows a speed control system wherein the regulating element and the motor are comprised in a single integral structure, the

regulating winding 6 being mounted on the rotor member together with the motor primary winding 12 which may be connected to the line throu 3p rings 13. brushes 1% and leads 15, am the secondary windings 1c of the motor being interconnected with a regulating trans mer 3' provided with adable taps 1T anal ltl anal wuu fixed taps where 2 is the number of poles. hen the adjustable ta )3 l7 coincide with the stationary taps l9, zero, the motor secondary windin 's U? are short circuited. and the machine we is an ordinary induction n'iotor. By moving the taps 17 from the taps 19 in one direction or the other. the speed of the motor may be varied between certain hypersyn: hronous and hyposynchronous values.

The pressure between the JlUSllCi. 8. in the arrangement of Fig. -l dependent on the voltage applied to the slip rii l3 and is independentofthe motor speed. i the speed of the motor changes, however the I of the current transmitted between the wind-- ing 6 and transformer 3 changes in a manner to vary the flux of the trzmsformen and it is desirable that the t: is 18 be ac in a manner to render the transfer-me substantially independent of frequency. At sy chronous speed of the machine. direct current is transmitted between the winding 6 and the transformer 3. In order to limit this current to safe values, the sectional area of the primary winding may be gradually increased as indicated in the figure or other means of limiting the regulating current in the neighborhood of synchronism may be provided. It will be apparent. that the autot-ransformer 3 may be replaced by a transformer having conductiyely independent primary and secondary windings in installations where the operation of the motor is not carried through syncbronism and that the windings 6 and 12 of the motor may be replaced by a single winding.

Fig. 5 shows a speed control system wherein the regulating machine 1 is arranged to be supplied with current from an alternator 20 mounted on the motor shaft and to be driven by a synchronous machine 21 which is supplicdwith alternating current from the polyphase line 4. suppl ed with direct current excitation through line 22, adjustable resistor 23 and slip rings 24, and is provided with a worm screw 25 for making angular adjustments in the position of its stator member. The alternator 20 may be provided with angularly displaced field windings, as shown. for example in United States Patent 1,529,341, having their terminals connected to the slip rings 26 and 27 respectively, the slip rings 26 and 27 being interconnected with the line 22 through resistors 28 and 29 respectively which may be adjusted independently to control the angular position of the alternator field and the phase of the voltage impressed on the motor 2 by the machine 1. The hase of this voltage may be controlled a so by shifting the brushes of the regulating machine 1 or by angular adjustments in the stator of the machine 21. The resistors 28 and 29 are simultaneously adjusted to vary the voltage of machines 1 and 20 for the purpose of controlling the motor speed.

The embodiments of the invention illustrated and described herein have been selected for the purpose of clearly setting forth the principles involved. It will be apparent,

, however, that the invention is susceptible of being modified in many ways, to meet the different conditions encountered in its use and I therefore aim to cover by the appended claims all modifications within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is

1. An induction motor speed control system comprising a wound secondary induction motor, a variable frequency source of excitation for the secondary of said motor, a trans former connected between said source and the secondary of said motor, means for adjusting the transformation ratio of said transformer for varying the voltage impressed upon the secondary of said motor, and means for varying the impedance of the primary circuit of said transformer independently of said first mentioned adjustment.

2. An induction motor speed control system comprising an induction motor having a secondary winding, a rotor core member provided with a commutated winding for applying a regulating voltage to the secondary circuit of said motor, a stator core member provided with slots arranged to limit the flux in the commutating zone of said winding, and a transformer arranged to have its transformation ratio adjusted for controlling the magnitude of said regulating voltage and to have the number of its primary turns adjusted independently of said first mentioned adjustment for preventing change in the transformer flux due to variation in the frequency of the current transmitted therethrough.

3. An induction motor speed control system comprising an induction motor having a secondary winding, a rotor core member provided with a commutated winding for applying a regulating voltage to the secondary circuit of said motor, a stator core member provided with slots arranged to limit the flux in the commutating zone of said winding, and a transformer arranged to have its transformation ratio adjusted for controlling the magnitude of said regulating voltage and to have the impedance of its primary circuit adjusted independently of said first men- HIDDE K. SCHRAGE. 

